1. Field of the Invention
The present invention relates to a fiber amplifier which provides high output with low noise and low distortion and a fiber transmission system using such a fiber amplifier.
2. Description of the Related Art
An optical communication system includes a semiconductor laser as a signal source for generating a signal, a fiber as a route for transmitting the signal, and a light receiver for detecting the signal. The system also includes a fiber amplifier when it is required to compensate for a transmission loss and a distribution loss. The optical communication system has two signal transmission methods; the analog signal transmission and the digital signal transmission. In the analog signal transmission, the noise characteristic and the distortion characteristic are major items for evaluation, while in the digital signal transmission, the code error ratio characteristic is a major item for evaluation.
The noise figure (NF) of a fiber amplifier can be determined by measuring the deterioration in the carrier-to-noise ratio (CNR) of emergent light with respect to that of incident light. This deterioration is mainly due to multiple reflection of amplified spontaneous emission (ASE) and signal light. It is reported that ASE noise significantly increases when reflection points exist in the fiber amplifier (IEEE Photonics Technology Letters, Vol. 4, No. 6, pp. 568-570, 1992). Also reported is that, in general, multiple reflection noise increases when reflection points exist on a transmission route (Journal of Lightwave Technology, Vol. 9, No. 8, pp. 991-995, 1991).
The fiber amplifier includes an optical fiber doped with erbium ions, one type of rare earth element ions, and an optical coupler for coupling pump light to the erbium-doped fiber. Optical isolators for suppressing light reflection inside the fiber amplifier are disposed at the input end and the output end of the erbium-doped fiber.
The placement of the optical isolators at the input and output ends of the erbium-doped fiber amplifier causes insertion loss and consequently deteriorates the NF and reduces the amount of output light. Further, using two optical isolators increases the cost since an optical isolator is expensive.
A fiber amplifier where optical isolators are not disposed at both the input and output ends of a fiber amplifier is reported (IEEE Photonics Technology Letters, Vol. 5, No. 2, pp. 232-235, 1993). However, since the fiber amplifier of this report is for amplifying digital signals, no indication is found on the NF characteristic and the distortion characteristic. Therefore, the analog signal transmission characteristics are unknown.
A structure where only one optical isolator is disposed in the middle of a rare earth element doped fiber is reported (IEEE Journal of Quantum Electronics, Vol. 31, No. 3, pp. 472-480, 1995). This report studies only the ASE noise, but not the characteristics including signal light multiple reflection noise observed when reflection points exist in the optical fiber. No optimization of the noise characteristic is reported. No indication is found on the distortion characteristic. Therefore, the analog signal transmission characteristics are unknown.
As described above, in the conventional fiber amplifiers where optical isolators are not disposed at both input and output ends of an optical fiber, the analog signal transmission characteristics at the amplification of a high-input sub-carrier modulation (SCM) signal have not been studied.
An optical isolator connected to an optical fiber to prevent multiple reflection generates a loss of about 0.5 dB (10%). The NF deteriorates when such an optical isolator is connected to the input end of the doped fiber. The output light amount lowers when it is connected to the output end of the doped fiber.